In the field of graphic art, a set of separated color images are prepared from a color original sheet using a lith type film, and a final color image sheet is prepared using the separated color images. Prior to the final printing, a color proof is generally prepared to check for any mistakes possibly introduced in the preparation of the set of separated color images and to further check whether color adjustment is required. A paper sheet is generally employed as the material for preparing the color proof because the color proof should be analogous to the finally printed paper sheet. For the same reason, a pigment is preferably employed as coloring material. Further desired is a high resolution so that a half tone is precisely reproduced. Furthermore desired is an enhanced reliability of the process.
Recently, there arises a demand for a process for preparing a color proof by a dry process, namely, a development process using no developing solution.
At the present time, the stage prior to the printing, namely, prepress, is highly computerized. Therefore, a process and material for directly reproducing a color proof from a set of digital signals is required. In such a computerized system, it is needed to produce a color proof of extremely high quality. Generally, an image of at least 150 lines/inch is required. For preparing a proof of such a high quality from digital signals, a laser light which is highly coherent and which can be modulated by digital signals should be employed as a recording head. Therefore, it is required to develop a recording material which shows high sensitivity to laser light and shows light resolution as to reproduce very fine dots.
Various recording materials employable for a transfer image forming method utilizing a laser beam are known. Japanese Patent Provisional Publication No. 5(1993)-58045 discloses a recording material comprising a support, a light-heat conversion layer thereon which absorbs a laser light and instantly produces a heat and an image forming layer, which is provided on the heat-light conversion layer, comprising pigment dispersed in a binder such as wax. The image forming method using the recording material comprises the steps of: applying a laser light imagewise the light-heat conversion layer from the support side to produce a heat in the light exposed area of the light-heat conversion layer, fusing the image forming layer in an area corresponding to the light exposed area by the heat, and transferring the fused image forming layer onto an image receiving sheet to form an image. Japanese Patent Provisional Publications No. 63(1988)-104882 and No. 4(1992)-208496 disclose a recording material comprising a support, a light-heat conversion layer and an image forming layer comprising sublimation dye dispersed in a binder. The image forming method using the recording material comprises the steps of: applying a laser light imagewise the light-heat conversion layer to produce a heat in the light exposed area, sublimating the dye in an area corresponding to the light exposed area by the heat, and transferring the sublimated dye onto an image receiving sheet to form an image.
Further, Japanese Patent Provisional Publication (for PCT application) No. 2-501552 discloses a recording material which is employable for reproducing an image of very fine halftone by means of the above-mentioned laser light which is highly coherent and can be modulated by digital signals. The recording material comprises a transparent support, an image forming surface layer, which turns fluidal upon receiving a heat, and an image forming material layer of porous or granular material. When the laser light is applied, the image forming material layer in the area exposed to the laser light is fixed onto the support. Then, the unexposed area of the image forming material layer is peeled off to leave an image formed of the exposed image forming materials layer on the support.
In the above image forming method, the image is formed directly on the transparent support. Therefore, the employable support is limited. Further, it is not easy to prepare an image of multi-color. Accordingly, this process is not appropriate for employment as a method for preparing a color proof which generally needs the use of paper sheet (i.e., pulp paper sheet) and on which a multi-color image is generally formed.
Japanese Patent Provisional Publication No. 6(1994)-219052 describes an image transfer sheet which comprises a support, a light-heat conversion layer of a light-heat conversion material, a thermally activable releasing layer of very small thickness (such as 0.03 to 0.3 .mu.m), and an image forming layer comprising a coloring material. In this image transfer sheet, the bonding force between the image forming layer and the light-heat conversion layer decreases in the area where a laser light is applied. Such a decrease of the bonding force is caused by thermal deterioration of the releasing layer. If an image receiving sheet is beforehand provided on the image forming layer when the laser light is applied to the image transfer sheet, an image of an area exposed to the laser light is transferred onto the image receiving sheet. In this system, the transfer of image is accomplished by so called "ablation". In more detail, the releasing layer decompose to produce a gas in the exposed area to the laser light, and hence the bonding strength between the light-heat conversion layer and the image forming layer decreases in that area. The image forming layer on that area is then transferred onto the image receiving sheet. The image forming system utilizing the "ablation" is favorable in that a paper sheet having an adhesive undercoat can be employed as the image receiving sheet and a multi-colored image with fine tone can be easily prepared by placing the image transfer sheets of different colors on the image receiving sheet by turns. Accordingly, this method is advantageously employable for preparing a color proof (particularly, DDCP: Direct Digital Color Proof) or an extremely fine mask image.
Each layer constituting an image transfer sheet employed in the above-mentioned image forming method is formed by the means of coating method, and therefore a coating liquid for forming each layer is desired to be suitable for coating operation or film-forming operation. For example, the light-heat conversion layer comprises a light-heat conversion material (generally dye capable of absorbing a laser light) and binder, and the binder is required to have high dispersibility for the light-heat conversion material and to have high heat resistance because the layer is heated to high temperature when a laser light is applied. Examples of binders contained in the light-heat conversion layer include acrylic homopolymer or copolymer of acrylic monomers such as (meth)acrylic acid or its alkyl esters; celluloses such as cellulose acetate; vinyl polymers such as polystyrene, vinyl chloride/vinyl acetate copolymer, polyvinylbutyral and polyvinyl alcohol; condensation polymers such as polyester and polyamide; rubber thermoplastic resin such as butadiene/styrene copolymer; polyurethane; polyimide; epoxy resin; and urea/melamine resin. These examples are, for example, described in Japanese Patent Provisional Publications No. 5(1993)-58045 and No. 6(1994)-219052 mentioned previously. Of these polymers, polyimide has high heat resistance. Polyimide is, however, hardly dissolved in an organic solvent and therefore it is difficult to prepare a coating liquid of polyimide. Thus, as polymers for the light-heat conversion layer, polyvinyl alcohol, polyvinyl butyral and polyester are usually employed.